Air quality in the United States has improved dramatically over the past 30 years as a result of the combined efforts of industry, individuals, and government. Approximately half of the U.S. population, however, still believes that air quality is deteriorating. According to a survey of the Foundation for Clean Air Progress, roughly 50 percent believe that air quality is worse than it was 10 years ago, 42 percent believe it is better, and eight percent believe it is the same.10

Air quality trends are measured in two ways: emissions and ambient levels. Emissions estimate the amount of material that comes out of a smokestack, automobile tailpipe, or other source. Emission estimates are typically measured in pounds or tons. Ambient levels refer to the actual concentration of a pollutant in the air, and are quantifiably measured through more than 600 U.S. sampling stations in parts-per-million or parts-per-billion.

While both emissions and ambient levels show significant decline over the past quarter-century, this report focuses on ambient levels. These measure the real exposure to pollution, from which health experts and scientists can discern the actual threats posed to human health, and also quantify potential environmental hazards.

Ambient concentrations depend not only on the amount of man-made emissions, but also on many meteorological factors such as temperature, sunlight, air pressure, humidity, wind, rain, and so forth. For example, hot summers, such as 1983 and 1988, experienced higher ozone levels, while cool summers (1992 was the second coolest summer in the United States during the last 100 years) experienced lower air pollution levels. The EPA is currently trying to develop models that will adjust for meteorological conditions to permit better trend analysis.

It should be noted that some air pollution, especially ozone-forming hydrocarbons and particulates, is naturally generated in substantial amounts by trees and other vegetation. These issues are discussed further in the analyses of the individual pollutants.

This report presents the latest national annual trend data from the Environmental Protection Agency (EPA) but the EPA's air quality data for 1998 were not available at the time of publication. The trend data for 1976 through 1997 can be found in the Pacific Research Institute's 1999 edition of the Index of Leading Environmental Indicators, available at www.pacificresearch.org.

As Table 1 shows, there have been major decreases in ambient levels of air pollution since 1976. The EPA notes, "Since 1970, total U.S. population increased 31 percent, vehicle miles traveled increased 127 percent, and the gross domestic product (GDP) increased 114 percent. During that same period, notable reductions in air quality concentrations and emissions took place."11